The sacroiliac joint is a synovial joint between the sacrum, which is the inferior (or caudal) terminus of the spinal column, and the ilium of the pelvis. As with other synovial joints, the sacroiliac joint (or “SI” joint) can degenerate and undergo degenerative arthritic changes due to a number of possible causes, including trauma to the joint or the hypermobility induced in the joint during childbirth. The degenerated SI joint loses stability and experiences non-normal movements, resulting in an inflammatory response and subsequent pain. It has been reported that disorders of the SI joint are the source of back pain for up to 25% of patients (Cohen, 2005).
It is accepted practice to treat certain degenerated synovial joints, which normally exhibit low relative motions between the joint surfaces such as the spinal vertebral disc and SI joint, by stabilizing the joint through immobilization and subsequent bony fusion (arthrodesis). It is thought that eliminating relative movement of the joint surfaces will eliminate subsequent inflammation and pain. Surgical arthrodesis of the SI joint was first reported by Smith-Petersen and Rodgers in 1926 and is still performed today. Arthrodesis is performed by inducing growth of bone between the joint surfaces to fuse the joint, first by removal of all of the soft tissue of the joint to eliminate any barriers to bone tissue formation, followed by scraping of the bony joint surfaces to induce bleeding and the subsequent biological response of bone formation, and often with filling the prepared joint with morselized bone to assist in the bone formation process. Immobilizing the joint as part of the arthrodesis procedure provides immediate stability to the joint and reduction of the pain-generating movement, and allows quicker bone formation.
Immobilization of the SI joint has historically been performed by placing one or more, simple threaded screws through the joint, normal to the general plane of the joint. Due to variations in bone density and/or morphology between individual patients, as well as occasional bone removal from the ilium from previous spinal fusion surgeries, simple threaded screws are often insufficient to provide the joint stability required for a successful arthrodesis procedure. There exists a need for improved designs that can offer improved ability to immobilize the SI joint in more patients and reduce the risk of back-out of the fixation device.